Chapter 32 Electrostatics

Introduction-

• Electricity in one form or another underlies just about everything around you from the lightning from the sky to the spark beneath your feet when you scuff across a rug
• Electrostatics-electricity at rest
• Involves electric charges, the forces between them, and their behavior in materials

32-1 Electrical Forces and Charges
· Electrical forces-pair of forces that are acting on you at all times· They arise from particles in atoms
· Unlike gravity which can only be an attractive force, the electrical force can either attract or repel.· For example, in the early 1900s, Ernest Rutherford and Niels Bohr constructed a model of the atom which a positively charged nucleus is surrounded by electrons
· Charge-an attracting and repelling behavior; no two charges that are the same can attract each other
o Protons are positively charged
o Neutrons neutrally charged (neither attract or repel any charge)
o Electrons are negatively charged

• Facts about atoms: every atom has a positively charged nucleus surrounded by negatively charged electrons
• All electrons are identical; that is, each has the same mass and the same quantity of negative charge as every other electron
• Nucleus is composed of protons and neutrons
• Atoms usually have as many electrons as protons, so this therefore means the atom has a zero net charge.
• Basic rule of electrical phenomena is—like charges repel; opposite charges attract

  This picture is demonstrating the effects of how like charges repel and opposite charges attract.

http://learn.uci.edu/media/OC08/11004/OC0811004_L6Charges.jpg












32-2 Conservation of Charge

• In a neutral atom, there are as many electrons as protons, so there is no net charge meaning that the total positive charge balances the total negative charge
• If an electron is removed however, the atom is no longer neutral
• Ion- charged atom
• Positive ion- net positive charge
• Negative ion-net negative charge
• If there is an imbalance in the numbers, the object is electrically charged, but if there is an equal number of electrons and protons, then there is no net charge
• The innermost electrons in an atom are very tightly bound where the electrons on the outer part of an atom are very loosely bound together.
• “An object that has unequal numbers of electrons and protons is electrically charged. If it has more electrons than protons, the object is negatively charged. If it has fewer electrons than protons, then it is positively charged.
• Electrons are neither created nor destroyed but simply transferred from one object to another. The total amount of charge, both positive and negative is constant. This is the principle of the conservation of charge.

32-3 Coulomb’s Law

• F=G m1 m2/d squared
• Between two objects of mass m1 and mass 2 is proportional to the product of the masses and inversely proportional to the square of the distance d between them:
• G is the universal gravitational constant
• Coulomb’s law-states that for charged particles or objects that are small compared with the distance between them, the force between the charges varies directly as the product of the charges and inversely as the square of the distance between them.
• F=k q1 q2/d squared
• D is the distance between the charged particles, q1 represents the quantity of charge of one particle and q2 the quantity of charge of the other particle; k is the proportionally constant
• SI unit of charge is 1 coulomb

32-4 Conductors and Insulators
· Conductors-electrons are more easily moved in some materials than in others, outer electrons of the atoms in a metal are not anchored to the nuclei of particular atoms, but free to roam in the material. These items are poor insulators.
· Metals are good conductors because their electrons are loose
· Insulators-electrons that are tightly bound and remain with particular atoms and cant wander about to other atoms in the material
· These objects are poor conductors.
· Ex: rubber and glass
· Semiconductors-can be made to behave sometimes as insulators and conductors.
· Superconductors- at temperatures near absolute zero, certain metals acquire infinite conductivity

32-5 Charging by Friction and Contact

• Electrons are being transferred by friction when one material rubs against another
• When a charged rod is placed in contact with a neutral object, some charge will transfer to the neutral object-charging by contact
• Good conductor=charge will spread, bad conductor=touch the rod in several places in order to get a more or less uniform distribution

32-6 Charging by Induction

• If a charged object is already available then it can be used to charge another object by induction
• You can do this by bringing another charged object such as a rubber rod near the object to be charged, allowing the displaced charge to drain off on the other side then removing the drain while the charged object remains ten finally removing the original charged object leaving the induced charge.

32-7 Charge Polarization

• Electrically polarized- when an insulator is brought near a charged object the charges in the insulator will rearrange slightly so that one side will have a slight positive charge and the other will have a negative charge.
• Example: A negatively charged balloon attracts molecules from the wooden wall and creates a positively charged surface. Therefore, the balloon will stick to the wall.
• Example: a charged comb will attract an uncharged piece of paper because the force of attraction for the closer charge is greater than the force of repulsion for the farther charge.

Review Questions:
1. Which force-gravitational or electrical-repels as well as attracts?
2. How do protons and electrons differ in their charge?
3. How do like charges behave?
4. How do unlike charges behave?
5. How does a negative ion differ from a positive ion?